In recent years, a battery-driven system has generally been employed in a computer system such as a notebook personal computer. In this battery-driven system, a driving time is limited because of correlation between a battery life and a driving time. For this reason, in the case of the computer system, a technology of extending a continuous driving time for a system by reducing power consumption for the system as a whole has been demanded.
With the current state of the technology, however, as power is always supplied to a sampling circuit for an A/D converter, the circuit is driven, even when an irregular signal is received or a period with no signal received continues, for sampling to subject the signal to A/D conversion. For this reason, even when it is conceivable from the view point of an A/D converter and a video signal processing circuit as a peripheral unit thereof or of the system as a whole that, even when the sampling driving is not needed, power is supplied to an A/D converter and the power is wastefully consumed, which makes an operating time of the whole system shorter.
There are some analogous technologies to solve the problem concerning the power consumption disclosed, for example, in Japanese Patent Laid-Open Publication No. HEI 5-176333 and Japanese Patent Laid-Open Publication No. HEI 6-292062. Disclosed in Japanese Patent Laid-Open Publication No. HEI 5-176333 is a technology that a video signal processing circuit detects a vertical blanking period from a received complex synchronizing signal and turns OFF an A/D converter for the period of time. Also disclosed in Japanese Patent Laid-Open Publication No. HEI 6-292062 is a technology as a method of saving power for a video camera that power to a camera block including an A/D converter is cut off during a standby state for recording.
In the conventional type of computer system like those in Japanese Patent Laid-Open Publication No. HEI 5-176333 and Japanese Patent Laid-Open Publication No. HEI 6-292062, only a prespecified period of time required for video signal processing like the vertical blanking period is devoted to reduction of power consumption, or reduction of power consumption is effected by certain operational situations in the system like the standby state for recording.
However, an approach of controlling power consumption by directly determining a received video signal itself is required for actually dealing with video signal processing in real time.